/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlinePartyInterface.h
Request for the game to fill in data to be sent with the join request for the leader to make an informed decision based on the joiner's state
typedef TMulticastDelegate_ThreeParams< void, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) &, const [FOnlinePartyId](API\Plugins\OnlineSubsystem\FOnlinePartyId) &, [FOnlinePartyData](API\Plugins\OnlineSubsystem\Interfaces\FOnlinePartyData) & > FOnFillPartyJoinRequestData